This invention relates to a communication network.
It is common for a host computer to serve multiple terminals, each of which might have local computing capabilities. A network connects the host computer to the terminals, and supports full duplex digital communication between each terminal and the host computer, and possibly among multiple terminals. Different kinds of networks are able to support different connection patterns among the nodes (host computer and terminals) connected to the network.
A typical television broadcast facility includes a studio equipped with at least one camera and a machine room containing multiple videotape recorders (VTRs). The producer in charge of a particular broadcast employs a control panel to select video signals from various sources, such as a camera in the studio and VTRs in the machine room, to compose the video signal that is to be broadcast. It is therefore necessary to deliver command messages from the control panel to the video sources to control the status of each video source, and response messages from the video sources to the control panel to indicate to the producer the status of each video source.
A bus standard was developed jointly by the Society of Motion Picture and Television Engineers and the European Broadcasting Union to facilitate control of multiple video devices such as cameras and VTRs, using a common bus over which messages can be passed to the devices. FIG. 1 illustrates in simplified form a network in accordance with the ESbus standard. As shown in FIG. 1, the network consists of a controller 2, which is typically embedded in the producer's control panel, and multiple tributaries 4 associated with respective video sources, indicated as a camera 10 and a videotape recorder 12. An additional tributary 14 is associated with the producer's control panel 16. The controller is able to transmit command messages in digital form to the tributaries over a bus 6 that is shown with solid lines, and the tributaries transmit response messages in digital form to the controller over a bus 8, shown with dashed lines.
A typical exchange of messages would begin with the control panel 16 issuing a command sequence to tributary 14. The controller 2 periodically executes a service poll over command bus 6, and tributary 14 responds to the service poll by forwarding the command sequence to the controller 2 over bus 8. If the command sequence is intended for camera 10, controller 2 forwards the command sequence to the tributary 4A, which forwards it to camera 10. Camera 10 executes the command and then formats a response sequence that is delivered to control panel 16 over the reverse route. For example, if the producer wishes the VTR to start playing and the camera to pan to a particular azimuth, he would operate the manual controls on the control panel 16 so that the control panel issues a PLAY command to be sent to VTR 12 followed by a PAN (.phi.) command to be sent to camera 10. The PLAY command is forwarded by the control panel 16 to its tributary 14. Upon a service poll by the controller 2 over command bus 6, the PLAY command is tagged with the address of the VTR's tributary 4B and is forwarded to the controller 2 over the response bus 8. The controller then issues a BREAK command, which causes all tributaries to monitor the command bus 6, addresses tributary 4B, and forwards the PLAY command to tributary 4B over the command bus when tributary 4B indicates that it is able to receive a command. Each unaddressed tributary ignores further messages until a subsequent BREAK-address sequence is received addressed specifically for it. Tributary 4B then passes the PLAY command to VTR 12, which executes the PLAY command, formats a status reply message PLAYING, and forwards this message to its tributary 4B. As before, tributary 4B awaits a poll and then forwards the PLAYING response to controller 2 over response bus 8, controller 2 issues the BREAK command and forwards the PLAYING response to the control panel tributary 14 over command bus 6, and finally tributary 14 forwards the PLAYING response to control panel 16 which provides a visual indicator of the new PLAYING status. In similar manner, the command PAN (.phi.) is sent from control panel 16 to tributary 14, over response bus 8 to controller 2, over command bus 6 to tributary 4A, and from tributary 4A to camera 10, and is executed. The reply message POSITION (.phi.) is sent from camera 10 to tributary 4A, over bus 8 to controller 2, over bus 6 to tributary 14, and from tributary 14 to control panel 16, which provides a visual indicator of the camera's new azimuth position.
FIG. 2 illustrates in schematic form a data router 18 comprising four source lines 20.sub.i (i=0-3) connected to respective sources 22.sub.i, four destination lines 24.sub.j (j=0-3) connected to respective destinations 26.sub.j, and a rectangular matrix of crosspoints 28.sub.ij that allow any source line to be connected to any destination line. As in the case of FIG. 2, i and j typically have equal ranges and the source 22.sub.k and destination 26.sub.k typically are the input and output sides of a communication port of the same device 30.sub.k. Thus, the router shown in FIG. 2 is able to support four devices 30. The router also comprises a controller 32 that is connected to each of the crosspoints for selectively controlling the states of the crosspoints.
In addition to the crosspoints and the controller shown in FIG. 2, the router includes input buffering to reduce input loading and provide a controlled termination for the incoming signal, and an output driver to clean up the signal and provide the energy that is necessary to transmit the signal over long interconnecting lines.
A router of the kind shown in FIG. 2 is able to provide either broadcast communication from, for example, source 22.sub.0 to any selected subset of destinations 26.sub.1 -26.sub.3, or full duplex communication between any two devices. In the broadcast mode, the controller 32 closes (renders conductive) a subset of the crosspoints in the horizontal row that is connected to the broadcasting source. In order to broadcast from source 22.sub.0 to destinations 26.sub.2 and 26.sub.3, the controller closes crosspoints 28.sub.02 and 28.sub.03. In order to provide full duplex communication between devices 30.sub.1 and 30.sub.3, the controller closes the crosspoints 28.sub.13 and 28.sub.31.
A large broadcast facility may have three or more studios and may also include edit bays. Although it would be possible for each studio and edit bay to be provided with a dedicated complement of all the machines it is expected to need, all connected to a network operating under the ESbus standard, it is not likely that each studio and each edit bay would be in use at the same time and that each would need all the machines that were available to it, and consequently there is a strong possibility that at any one time a significant number of machines would not be in use. It is therefore more economical to provide a central pool containing sufficient machines to meet the maximum expected requirements of the facility, and to reassign them among users (studios and edit bays) as needed. It would be convenient to be able to use a router to establish connection networks for the machines of a large broadcast facility, since by selecting the crosspoints that are closed, the network could be reconfigured to reassign machines as required. However, the ESbus standard requires that the controller be able to receive messages from any tributary, and this would require a router in which multiple crosspoints on a controller's destination line could be closed. Routers that are known to the applicants do not allow more than one crosspoint on any given destination line to be closed. Therefore, a given destination cannot receive messages from more than one source without reconfiguring the router.